XIIl] FEMALE GENERATIVE ORGANS 181 



creasing in size until about half way through the period and 

 maintains its functional activit}' till at or near the time of par- 

 turition, whereas in the non-pregnant ewe after oestrus the 

 organ continues to develop for only a few days before it becomes 

 smaller. Mature (or almost mature) Graafian follicles are not 

 associated ^\ ith corpora lutea, and the latter organs when present 

 appear to exercise a dominating influence upon the ovarian 

 metabolism and to inhibit follicular growth or even to promote 

 follicular atrophy. Consequently it is important in animals which 

 come 'on heat' at short intervals that the corpus luteum should 

 not persist for more than a short period, and this suj^iiression of 

 the corpus luteum in the non-occurrence of pregnancy is generally 

 characteristic of pol^oestrous mammals. In monoestrous animals 

 between the heat periods and in all animals under a condition of 

 pregnancy it is obviously not disadvantageous if ovulation or the 

 ripening of the Graafian follicles is partially inhibited during 

 these times when coitus does not occur. Correlated with the 

 shortening in the period during ^\hich the corj)us luteum persists 

 (i.e. in polyoestrous animals) there is no pseudo-pregnant de- 

 velopment of the uterus and mammary glands. Even in 

 polyoestrous animals, however, the corpus luteum may ab- 

 normally persist and cause sterility, and Williams recommends 

 that when this happens with cows the corpus luteum should 

 be squeezed out from the ovary, and states that when this is 

 done the oestrous cycle starts again forthwith, the cows very 

 soon coming 'in use' after the operation of removal. 



The cow like the sheep is polyoestrous, but the dioestrous 

 cycle lasts for 21 da^-s. The precise conditions which determine 

 the duration of the sexual season are little understood, and in- 

 vestigation into this subject is urgentl}' needed with the view of 

 controlling the l)ulling times of heifers and cows so as to regulate 

 the periods of calving and the suj^ply of milk throughout the jear. 

 The prooestrous changes like those of the ewe are relatively slight 

 and external bleeding from the vaginal opening does not usually 

 occur. There is sometimes a mucous discharge but even this may 

 be absent, and yet a normal oestrus may suiaervene. Owing to 

 this circumstance some 'cowmen' will let a 'period' pass bj% and 

 so lo.se three weeks, and possibly the chance of getting an animal 

 in calf for that season, simply because they failed to detect any 

 of the signs of the onset of 'heat.' Dioestrous cycles (in the non- 



